Chemo-Mechanics of Functional Thin Films for Lithium-Ion Batteries and Neuromorphic Computing Devices

Abstract

This thesis investigates chemo-mechanics of functional thin films for lithium-ion batteries and neuromorphic computing devices. We provide synthesis details of several functional thin films, including textured V₂O₅, textured VO₂, epitaxial VO₂ (020), epitaxial VO₂ (4̅02) and analyze their evolution of mechanical, structural, and chemical properties when subjected to external stimuli. Regarding batteries, we focus on two cathode systems: Li-S composites and sputtered V₂O₅ thin films. We monitor the evolution of stresses in both systems during electrochemical cycling and link these stresses to structural and morphological evolution. These studies provide insight into mechanics-based issues in high-capacity cathodes with an eye toward strategies that mitigate mechanical degradation and thus extend cycle lifetimes. Regarding neuromorphic computing devices, we investigate sputtered VO₂ thin films in the view of crystal structure and corresponding effects on mechanical behavior. We conduct in-operando measurements of stress and nanoindentation in VO₂ thin films as they undergo metal-insulator phase transitions during thermal cycling. We observe that tensile stresses develop in the film upon heating through the phase transformation, which is somewhat counterintuitive given the known volumetric expansion associated with this transformation. We explain this phenomenon through structural analysis. The experiments also indicate a critical film thickness (of around 600 nm) above which polycrystalline VO₂ thin films will fracture. A corresponding fracture mechanics analysis of thin films captures this observed phenomenon. Overall, our detailed mechanical investigation can provide guidance towards practical implementation of mechanically-robust VO₂ into devices.